Old Window Glass Does Not Slowly Drain Downward
The claim that old windows are thicker at the bottom because glass has flowed there over centuries confuses glass’s disordered structure with liquid behavior. Ordinary window glass is an amorphous solid: its atoms lack the orderly repeating pattern found in a crystal, but they are locked into a rigid structure at room temperature. They do not rearrange quickly enough for a pane to sag under its own weight.
Glass can flow when it is heated, which is why it can be shaped in a furnace. But at everyday temperatures, its resistance to changing shape is so great that the slow movement imagined in the old-window story is not a practical explanation for uneven panes.
Why Historic Panes Can Be Uneven
Older glassmaking methods often produced sheets that varied in thickness. In some processes, makers spun molten glass into a disk and cut panes from it; elsewhere, they formed glass by blowing and flattening it. These methods could leave ripples, bubbles, or thickness variations. Glaziers sometimes put the thicker edge at the bottom when fitting a pane, but that choice does not mean the glass flowed there after installation.
The familiar appearance of a lumpy old window is real; the proposed cause is not. Its irregularities are generally traces of how the glass was made and installed, preserved in place because the material stays rigid under ordinary conditions.
A Solid Without a Crystal Pattern
Calling glass a solid does not require its atoms to form neat rows. The useful distinction is whether the material holds its shape on the timescale that matters. A window pane does. Heat it enough, and its structure becomes mobile so it can be worked; leave it in a building, and its uneven edge remains a record of its manufacture—not a slow-moving pool.
